1. Field of the Invention
This invention relates to an vehicle steering control device provided with an angle superimposing unit for variably controlling a road wheel angle with respect to a steering angle of a steering operated by a driver and an electric power assist unit for assisting a steering torque of a driver.
2. Description of the Related Art
There has been proposed an vehicle steering control device provided with an angle superimposing unit which variably controls a ratio between a steering angle and a road wheel angle by superposition an intervening angle by an electric actuator on the steering angle operated by a driver with the use of a planetary gear mechanism and a hydraulic power assist unit which assists a steering angle of the driver (see, for example, Patent Document 1).
In this device, it is possible to replace the hydraulic power assist unit with an electric power assist unit.
Though the steering control device provided with the electric power assist unit and the angle superimposing unit has a torque sensor since an assist torque of the electric power assist unit is decided based on the steering torque of the driver, it is impossible to decide the assist torque when the torque sensor develops trouble, thereby making it difficult to continue the assistance.
Therefore, there has been proposed an electric power assist unit which has a torque sensor of a double system type including a main torque sensor and a sub torque sensor from the standpoint of failsafe design and is capable of continuing torque assist by switching to the sub torque sensor in the case where the main torque sensor is abnormal due to trouble or the like (see, for example, Patent Document 2).
Patent Document 1: JP-A-9-11928 (pages 3 to 6, FIGS. 1 to 9)
Patent Document 2: Japanese Patent No. 3390333 (pages 5 to 6, FIG.3)